Not an airtight plot, and not an entirely original one. Off the top of my head I can think of one film, 'Time Lock', five years earlier (1957), which had a similar premise (i.e. - getting someone out of a bank vault before they suffocated); there were probably others too. And in the end there are a couple of coincidences and contrivances too many to give it full marks. Still, it is way above your average forgotten cheapie.
Vernon Sewell had already made more than twenty features by this time, including the excellent 'The Man In The Back Seat' (also with Derren Nesbitt and Keith Faulkner), and experience shows in a tight framework which is hardly allowed to slacken for a moment. Much depends on performances, and all four main leads are superb. The script too is as strong as a fist, with a real punch at the end. The straight-laced bank manager's unbuttoning of his private life, his thwarted hopes and ambitions, at the same time as jacket, tie and collar are loosened, could not have been better done.
When a film makes you sympathise with the baddies as well as the innocent, it is doing something unusual, and genuinely remarkable.